Field of the Invention
The present invention relates to the semiconductor field, and more specifically, to a method for bonding two wafers.
In integrated circuits traditional dielectric insulation has been used for decades whenever direct electrical isolation between individual components is indispensable. Such direct electrical insulation can extend to an integrated circuit or a component being dielectrically totally insulated from another integrated circuit or another component. This means that the respective integrated circuits or components in a substrate are totally isolated from neighboring integrated circuits or components by means of dielectric insulation. Consequently, such total dielectric insulation necessitates horizontal dielectric insulation on the side toward the substrate, the side being opposite to the surface of the integrated circuit, and vertical dielectric insulation, which leads from the horizontal dielectric insulation to the surface of the substrate and surrounds the integrated circuit or the component.
Integrated circuits or components which are dielectrically insulated in this way are used, by way of example, in switching technology, in military applications which demand high radiation resistance, and increasingly in power electronics as well.
Specifically in order to produce the horizontal insulation, a so-called handle wafer and a wafer are bonded directly to one another, one or both wafers are provided with an oxidation layer on the surfaces to be bonded. This procedure is referred to as the "DWB method" (Direct Wafer Bonding) and has been used for a long time: the two wafers, which are as smooth as possible and are oxidized on one or both sides, are placed on top of one another and subjected to a heat treatment at 1000 to 1400.degree. C., preferably 1200.degree. C. Atomic bonds of greater or lesser strength are evidently formed between the two wafers in the course of this heat treatment.
The wafer into which the integrated circuits or components are to be introduced is then thinned by grinding. Vertical insulation is applied by first etching trenches in from the surface of the thinned wafer, and these trenches reach as far as the oxide layer at the boundary between the two wafers bonded to one another. These trenches are finally filled with silicon dioxide or polycrystalline silicon.
In this way, it is possible to produce individual monocrystalline silicon islands in the thus "active" wafer, which islands are totally dielectrically insulated from one another.
The production of such DWB wafers from two wafers bonded to one another with a diameter of 6" is very complicated and gives rise to costs which are approximately an order of magnitude higher than the costs of an individual standard wafer. The cause of this high outlay is found in the low yield during the production of the DWB wafers: the adhesion between the wafers bonded one on top of the other is difficult to control, flexure of the individual wafers occurs, and detachment of individual silicon islands in the active wafer from the handle wafer may be observed.
In order to overcome these difficulties, special attention has been directed heretofore at configuring the bonding surfaces of the two wafers to be as smooth as possible. Although considerable progress has been achieved in that area, a decisive breakthrough has been unattainable to date. The problems evinced above have not been able to be solved to date.